1. Field of the Invention
The present invention relates to a method of controlling the pulverization and drying of flammable materials, such as coal, in a system including a pulverizer. The invention also relates to a method of controlling the pulverizing rate of the pulverizer over a wide range.
Generally, in the case of pulverizing a flammable material such as coal, there is adopted a method wherein a high temperature dry gas for removing moisture contained in the material is fed into a pulverizer to preheat and dry the pulverized coal and at the same time the pulverized coal is conveyed pneumatically to a predetermined place by the dry gas. In the case of practicing such a method, there is a danger of the pulverized coal exploding in an atmosphere wherein the oxygen concentration is in the range of 10 to 13% or higher. Therefore, in order to avoid such a danger, it is necessary that the oxygen concentration of the dry gas be held below the concentration at which the coal dust may explode.
Therefore, an inert gas or an exhaust combustion gas is usually used as the dry gas, and in many cases, it has been mixed with a temperature regulating air or exhaust gas having an oxygen content in a range below the above-mentioned oxygen concentration.
As possible methods of operation in such a pulverizing and drying system, there are the positive pressure method for maintaining the interior of the system at a positive pressure and the negative pressure method for maintaining it at a negative pressure. In the former, the operation and control are performed by a push-blower mounted at the inlet of the system for pushing dry gas into the system. In the latter, a pull-blower is mounted at the outlet of the system for sucking out dry gas. However, these methods involve the following problems, and have been unsatisfactory. That is, the positive pressure method creates a positive pressure in the system such that the dry gas and the pulverized coal may leak out of the system, resulting not only in a lack of oxygen in the surrounding environment due to the dry gas, but also a danger of a secondary explosion due to the leakage of the pulverized coal. The negative pressure method is also disadvantageous in that air is likely to leak inside of the system through the rotating and piercing portions of the pulverizer, or the coal supply equipment, thus resulting in increased oxygen concentration within the system, which may cause an explosion of coal dust within the pulverizer.
The pulverized and dried coal is typically supplied to a boiler having a feedback control based upon the demand of the boiler for the pulverized coal as fuel. The amount of pulverized coal required is not constant over time and, as the case may be, the minimum demand is as low as 1/3 or less than the maximum demand. Therefore, in operating the pulverized coal preparation system, it is necessary to construct a system so that the coal feed rate is automatically controlled by the appropriate feedback from the boiler, or other final use.
In the conventional system, the pressure drop or differential pressure across the pulverizer was measured, and the rate of coal feed into the pulverizer was modified in accordance with this pressure drop. Thus, the control of the feed into the pulverizer in the conventional system was performed so that the amount of coal remaining in the pulverizer was always constant, since the pressure drop across the pulverizer was proportional to the amount of coal within the pulverizer. During normal operation, a differential pressure through a loop connected between the inlet end and the outlet end of the pulverizer was measured and its deviation from a preset differential pressure value was determined. The coal feed rate was controlled by this deviation. Where it was desired to positively change the coal pulverizing rate, it was necessary to change the preset value of the differential pressure to match a preset value corresponding to the desired coal pulverizing rate. The resulting differential pressure deviation then acted upon the controller for the coal feed and varied the coal feed rate.
However, in such a conventional system, the control of the coal pulverizing rate was made, not by regulating the pulverizing motor speed of the pulverizer, itself, but by simply adjusting the coal feed rate in accordance with the increase or decrease of the amount of raw materials staying in the pulverizer. Therefore, the controllable range for the coal pulverizing rate was narrow and the ratio of the maximum pulverizing coal rate to the minimum coal pulverizing rate was only about 1.5 to 2, at most.